Time varying wall shear rate is measured in the canine descending thoracic aorta in vivo. A hot film anemometer and miniature flat ended hot film probe are utilized in this measurement. Positive and negative components of the shear rate waveform are discriminated by blood temperature measurements utilizing a second thin film mounted onto the hot film probe. Thus, shear rate and blood temperature can be measured simultaneously at the same location on the aortic wall. These dual measurements will require separate calibration procedures to relate anemometer bridge voltage to shear rate and blood temperature, respectively. Blood temperature data is utilized to correct wall shear rate data during periods of flow reversal. Pulse echo ultrasound is utilized to monitor the position of the hot film probe tip relative to the surface of the aortic wall. This information can aid in monitoring flush conditions between the hot film probe and aortic wall. Blood viscosity is computed from blood hematocrit measurements. Wall shear stress can then be computed from wall shear rate and viscosity data utilizing a constitutive model for blood rheology. Both peak and mean wall shear stress levels are to be determined for twenty dogs in order to correlate in vivo data to shear stress values which have been reported to enhance transendothelial transport of macromolecules, a precursor to atherosclerosis.